A thermoplastic laminate comprising a film and a foam lamina of similar thermoplastic materials can be formed by a blown bubble coextrusion process. In such process, the film lamina is extruded from the outer annular orifice of a coextrusion die while the foam lamina, consisting of the thermoplastic material mixed with a foaming agent, is simultaneously being extruded from the inner annular orifice of the coextrusion die to form a tubular laminate. As the tubular extrusion leaves the coextrusion die, it is expanded by air under pressure and drawn over a sizing mandrel having diametrically opposed knives which slit the tubing longitudinally on opposite sides thereof to form two separate sheets of laminate. During the coextrusion process, it is desirable to control the average thickness of the film lamina of the laminate independently of the thickness and density of the foam lamina. This is usually accomplished by examining a cross section of the laminate under a microscope to determine the thickness of the film lamina and then, based on the examination, adjusting the rate of extrusion for the film lamina to achieve the desired thickness. Independent measuring was necessary because conventional gauging apparatus could not independently measure the thickness of the film lamina since both laminae were formed from the same or similar thermoplastic materials. For example, a conventional infrared gauge is ineffective because the absorption spectra of the two laminae are similar. Conventional inductive gauges are also ineffective because it depends on an underlying conductive lamina. Conventional capacitive gauges alone cannot distinguish between laminae.